1. Field of the Invention
The present invention generally relates to CDMA receivers and to blind adaptive equalizers for a multiuser CDMA communication system.
2. Related Art
Recently Code Division Multiple Access (CDMA) telecommunication systems have gained ground and popularity against rival TDMA/FDMA solutions, despite the increased complexity of the receiver. In the CDMA framework, different users employ distinct spreading codes but transmit at the same time and frequency. CDMA systems break up digital communications into bits at the sending end, and spray the bits across a slice of the spectrum. At the receiving end, the bits are reassembled based on signatures, or wave forms, associated with the bits of the corresponding users. A problem with CDMA systems arises because of interference from other users. This is especially prevalent where there are weak signal users and/or there are reflections from buildings, mountains or other structures that distort signals (multi-path distortions). Various interference suppression schemes have been used in the past to separate out Multi-User Interference (MUI) to obtain a clean signal at the receiving end. In one such scheme, signals are filtered out, one by one, from the strongest to the weakest.
Significant effort has focused recently on designing multiuser receivers in order to suppress MUI and deliver the promised capacity gains of CDMA technology. While optimal solutions have been derived, their exponentially increasing complexity renders them inappropriate for systems with a large number of users. For this reason, recent efforts have concentrated on suboptimal linear solutions and have investigated ways to derive the receiver parameters from the data.
Multiuser receiver design can be divided into two categories based on whether training sequences are used or not. In the first category, non-blind MMSE receivers can be adaptively implemented if the desired signal is known at the receiver, while in the second category, a blind approach has to be employed. Blind subspace methods show good performance, but the required SVD of large matrices leads to a heavy computational load. On the contrary, blind adaptive multiuser detectors significantly reduce the computational burden. A scheme was presented by Honig, et al., xe2x80x9cBlind Adaptive Multiuser Detectionxe2x80x9d IEEE Transactions on Information Theory, Vol. 41, No. 4, pp. 944-960, July 1995, for the case when multipath interference is absent and showed performance equivalent to that of the MMSE receiver. In Wang, et al., xe2x80x9cBlind Multiuser Detection: A Subspace Approachxe2x80x9d IEEE, Tans. On the Information Theory, Vol. 44, No.2, pp. 677-690, March 1998, an adaptive implementation based on subspace tracking method was shown to improve performance at the expense of more computational complexity. The method of Honig, et al. was later extended by adding more constraints by Schodorf, et al., xe2x80x9cA constrained Optimization Approach to Multiuser Detectionxe2x80x9d IEEE Transactions on Signal Processing, Vol. 45, No. 1, pp. 258-262, January 1997. Unfortunately, in the presence of multipath and multichip interference, minimum variance solutions are known to be sensitive to signature mismatch. A solution for that case was attempted by Tsatsanis, xe2x80x9cInverse Filtering Criteria for CDMA Systems, IEEE Trans. On Signal Processing, Vol. 45, No. 1, pp. 102-112, January, 1997, and later by Shen, et al., Blind Adaptive Multiuser CDMA Detection Based on a Linear Projection Constraintxe2x80x9d Proc. SPAWC""97, pp. 261-264, La Villette, Paris, France, Apr. 16-18, 1997, by forcing the receiver response to delayed copies of the signal of interest to zero. With these additional constraints, minimum variance techniques are applicable, but have inferior performance since they treat part of the useful signal as interference. Recently however, constrained optimization solutions were developed which combine all multipath components of the signal of interest and jointly minimize MUI while maximizing the signal component at the receiver output, Tsatsanis, et al., xe2x80x9cPerformance Analysis of Minimum Variance CDMA Receivers,xe2x80x9d Proc. 13st Intl. Conf. On Digital Signal Processing (DSP97), Vol. 1, pp. 379-382, Santorini-Hellas, Greece, Jul. 2-4, 1997. A related development from a different viewpoint was also reported in Liu, et al., xe2x80x9cLow Complexity Receivers for CDMA Communications over Frequency-selective Channels,xe2x80x9d Proc. SPAWC""97, pp.265-268, La Villette, Paris, France, Apr. 16-18, 1997. These blind methods exhibit superior performance which is close to that of the trained MMSE receiver Tsatsanis, et al. Unfortunately however, adaptive implementations of the solution of Tsatsanis, et al. are not straightforward, as they correspond to the optimization of a non-linear cost function.
Other efforts in this area include:
Kawabe, et al., U.S. Pat. No. 5,394,434 multiple-access (CDMA) demodulator which stores received chip data in a separate memory for each transmitting station. Each memory area holds data for a plurality of symbols. When the receiving station finishes receiving a signal from one transmitting station the corresponding area is designated for processing. All data stored in the designated area is correlated with the spreading code of the transmission station to generate correlated values for a plurality of symbols. Estimated symbols are derived from the correlated value and the temporarily stored. The difference between each estimated symbol value and the previous estimate is multiplied by the spreading code of the designated station to generate remaining interference value. The remaining interference values are subtracted from all chip data stored in the memory except for data stored in a designated area thereby updating the memory contents. A demodulated value is output for the oldest symbols stored in a designated memory area.
Divsalar, et al., U.S. Pat. No. 5,644,592, discloses a method of decoding a spread spectrum composite signal to provide an improved method of interference cancellation for CDMA communications. The system comprises a parallel interference cancellation (IC) system that reduces the degrading effect of multiuser interference. The patent discloses a method of decoding a spread spectrum composite signal comprising plural user signals spread with plural respective codes, wherein each user signal is despread, filtered to produce a signal value, analyzed to produce a tentative decision value, respread, summed with other re-spread signals to produce combined interference signals, combining a portion of each signal value with one of the combined interference signals and the composite signal to produce an estimate of a respective user signal to preserve information contained in the user signal. Each signal is processed for each user by a matched filter which would be dominated by the corresponding user signal and will produce a maximum signal to noise ratio (SNR). Thereafter, a re-scaling amplifier amplifies the signal with an estimate of the transmitted power. The method attempts to cancel the multiuser at each stage of the IC device.
Hoff, et al., U.S. Pat. No. 3,619,586 discloses a second-recuversive digital filter optimized by the equation
f(n)=CX(n)+2(%+xcex22)f(nxe2x88x921)xe2x88x92f(nxe2x88x922)+xcex21[%+xcex22)f(nxe2x88x921)xe2x88x92f(nxe2x88x922)]
where the sum of %+xcex22 may be provided as a single coefficient K. The filter can be used as a band-pass filter or as a oscillator.
Tanaka, et al., U.S. Pat. No. 5,602,765 discloses an adaptive transfer function estimating method for estimating with a projection algorithm function of an unknown system and its output in an acoustic cancelier, active noise control, or the like. The estimated transfer function correcting vector calculation calculates an estimated transfer function correcting vector on the basis of the error signal and the input signal to the unknown system.
Weng, et al., U.S. Pat. No. 5,652,903 discloses a co-processor for use with a digital signal processor used on an integrated circuit to provide multiple communication functions. The device includes a database interface for receiving input samples and algorithm co-efficients from memory, a sequencer for storing a processing algorithm, an internal memory for storing intermediate results, a data core including an address generation and arithmetic unit for executing operational instructions of the signal processing algorithm to produce resultant signals from input samples.
Sugimoto, et al., U.S. Pat. No. 5,579,304 discloses a code division multiple access (CDMA) receiver which operates by despreading code of the relevant station and estimating the value of the symbol. Then the spreading code is used to estimate interference signals and the baseband signal is modified by subtracting the estimated interference signal. The canceled interference from the baseband signal corresponds to the information in the estimated symbol value. The method is repeated successively from one stage to the next.
Fukasawa, et al., U.S. Pat. No. 5,463,660 discloses a block-spreading CDMA communication system comprising a modulator for use in a receiving station for receiving a combined signal from a plurality of transmitting stations. The received signal yields baseband chip data that is stored in memory. When the receiving station receives a block from a transmitting station, it designates that transmitting station and the corresponding memory area for processing. Each data block stored in the designated memory area is correlated with a set of product codewords obtained by spreading all codewords in a set of orthogonal codewords by the spreading code of the designated transmitting station, and the product codeword that gives a maximum correlation value for that block is selected. For each block, the selected product code and maximum correlation value are used to calculate a set of interference correction data which data is subtracted from the corresponding chip value stored in the other memory areas to update all data stored in the memory except for the data stored in the designated memory area.
Yates, et al., U.S. Pat. No. 5,307,341 discloses a frequency comb multiple access (FCMA) communications system in which address and message data are transmitted on a common channel to different users. The data includes signatures corresponding to an address and a data message directed to the address. The invention provides a coding structure to minimize overlap between user signatures and accordingly, minimize errors between different users.
Falconer, et al., U.S. Pat. No. 5,204,874 discloses a method and apparatus for encoding and decoding in a communications system. In decoding, received data samples are grouped and transformed according to one of two algorithms, one which generates a plurality of soft decision similarity metrics and index data symbols for each group, and one which consists of deinterleaving by group each group of data samples within a predetermined size block and subsequently generating a plurality of soft decision similarity metrics and index data symbols. After the group samples are transformed, at least one estimated data bit is generated by utilizing maximum likelihood decoding techniques to derive at least one data bit from the index data symbols and associated soft decision metrics.
Taylor, U.S. Pat. No. 5,084,900 discloses a system including an inbound satellite communication link from terminal stations to a hub station for sending digital data packets encoded with a CDMA code for code division multiple access during synchronized time slots on a contention channel. Each of the digital data packets is encoded with the same CDMA code during a first transmission, but each packet is encoded with an alternate CDMA code during a subsequent transmission.
Further works include those by Comon, et al., xe2x80x9cTracking a Few Extreme Singular Values and Vectors in Signal Processing,xe2x80x9d Proc. of the IEEE, Vol. 78, No. 8, pp. 1327-1343, August 1990; Frost, xe2x80x9cAn Algorithm for Linearly Constrained Adaptive Array Processing,xe2x80x9d Proc. of the IEEE, Vol. 60, No. 8, pp. 926-935, August 1972; Haykin, xe2x80x9cAdaptive Filter Theory,xe2x80x9d 3rd Edition, Prentice Hall, Upper Saddle River, N.J. 1996; Johnson, et al., xe2x80x9cArray Signal Processing: Concepts and Techniques,xe2x80x9d Prentice Hall, Englewood Cliffs, N.J. 1993; and Van Veen, xe2x80x9cMinimum Variance Beamforming,xe2x80x9d Adaptive Radar Detection and Estimation, John Wiley and Sons, Inc., 1992.
The entire disclosure of these efforts of others is expressly incorporated herein by reference. None of these previous efforts, taken alone or in combination, teach or suggest all of the elements of the present invention, nor do they teach or suggest all of the benefits thereof.
It is an object of the present invention to significantly reduce multi-user interference in a CDMA system.
It is an additional object of the present invention to maximize the signal component of a desired signal at the receiving end of a CDMA system.
It is a further object of the present invention to provide a filter means for the receiving end of a CDMA system which is blind and adaptive.
It is even a further object of the present invention to provide blind, adaptive equalizers for a multi-user CDMA system having reduced complexity by employing constrained optimization techniques.
It is still yet another object of the present invention to provide blind, adaptive algorithms for filtering multi-user and multi-path interference.
It is even another object of the invention to provide an algorithm that provides improved results each time it runs.
It yet another object of the present invention to provide a method which operates in multi-path environments by accounting for multi-path components of the signal of interest.
It is yet another object of the present invention to provide a blind, adaptive algorithm for use in a CDMA receiver which recursively minimizes the output variance of the received signal subject to certain constraints which are also jointly updated.
The present invention utilizes a filter that works on an algorithm that filters out multiuser and multi-path interference. The algorithm is programmed into the DSP processors of receiver handsets and base stations in cellular and multihop wireless radio networks which use the CDMA technology. It can also be implemented in hardware. Additionally, the invention works in a blind manner such that it learns without requiring training samples. The algorithm can be expressed as updating the receiver parameter vector f(n) as:
f(n+1)=[Ixe2x88x92C(CHC)xe2x88x921CH][f(n)xe2x88x92mfy(n)yH(n)f(n)]+C(CHC)xe2x88x921g(n)
wherein y(n) is a data vector for each transmitted bit W1(n) from user 1, the receiver will estimate that bit by filtering y(n), i.e.
w1(n)=fH(n)y(h),
where H denotes Hermitian; g(n) corresponds to the multipath parameters and is updated by a separate revision; and C is a matrix constructed from elements of the desired user""s spreading code. Importantly, the filter based on this algorithm is adaptive and can be used blind, i.e. no training of the device is necessary prior to operation, rather, the device is improved each time the algorithm is run.